High silicon niobium casting alloy and process for producing the same

ABSTRACT

An iron-based high-silicon alloy contains (in weight percent) 2.6-3.5% carbon, 3.7-4.9% silicon, 0.45-1.0% niobium, up to 0.6% manganese, up to 0.02% sulfur, up to 0.02% phosphorus, up to 0.5% nickel, up to 1.0% chromium, up to 0.1% magnesium, and the balance iron and up to 0.2% of other elements. The alloy is heat resistant and is suitable for use in producing, among other things, turbochargers, center housings, back plates, exhaust manifolds, and integrated turbo manifolds that are used in the automotive and truck manufacturing industries.

FIELD OF THE INVENTION

This invention relates generally to iron-based casting alloys andparticularly to those having high silicon content. It also relatesgenerally to processes for producing such alloys. More specifically, itrelates to an improved iron-based, high silicon niobium alloy thatdemonstrates enhanced high temperature strength and performancecharacteristics. It also specifically relates to the process forproducing this improved alloy.

BACKGROUND OF THE INVENTION

In the art of producing iron-based ductile alloys that are castable,there are certain end-product applications that require the use of aniron-based alloy that yields enhanced high temperature strengthend-products. Such end-products are used in a wide range ofapplications, one of those including “hot-side” engine parts. Typical ofsuch parts are turbochargers, center housings, back plates, exhaustmanifolds, and integrated turbo-manifold components that are used in theautomotive and truck manufacturing industries. As with any product inthe automotive industry, the market for such products is quite large andthe number of products that are required to be produced isproportionately large.

Molybdenum and niobium (also somewhat archaically known as “columbium”)are alloying elements that are known in the art. Niobium is currentlybeing used in the production of heat resistant stainless steels andaircraft engine parts. Molybdenum is also used in similar applications,but at a greater cost. Because niobium adjoins molybdenum in theperiodic table, these elements have very similar atomic weights. Theproduct of the present invention was intended to utilize niobium in sucha way as to provide a high-silicon niobium ductile iron with acceptableheat-resistance properties with reduced cost in mind. That is, sincelarge numbers of hot-side engine parts are used in the automotiveindustry, achieving sufficient high temperature strength while usingniobium in place of molybdenum would contribute to reducing the cost ofproducing such parts. During testing, however, it was found that thealloy of the present invention not only met the requirement of achievingsufficient high temperature strength, but actually exceeded thatrequirement and ended up providing a unique high-silicon niobium ductileiron with enhanced heat-resistance characteristics, and with a probablesaving of cost.

Another goal of the product of the present invention was to utilizeniobium in such a high silicon casting alloy wherein existingindustry-wide specifications and performance standards would be adheredto. More specifically, current high silicon molybdenum ductile alloyscalled out specific ranges for levels of certain elements to be used inthe alloy and that the alloy would possess certain minimum performancecharacteristics following casting. This inventor was of the view thatniobium could be used in a high silicon niobium alloy, at a savings ofcost, while preserving the required performance characteristics thatwere dictated by the industry. Not only did this view prove to be true,but performance characteristics were found to be enhanced.

Still another goal of the product of the present invention was toutilize niobium in an ultra high silicon casting alloy wherein corrosionand oxidation resistance characteristics were improved. That is, wherethe addition of chromium in ultra high silicon molybdenum alloys resultsin improved oxidation and corrosion resistance, this inventor was alsoof the view that niobium could be used in an ultra high silicon andchromium ductile iron in place of molybdenum without any degradation ofthose characteristics. This view proved to be true and with performanceactually being enhanced as well.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide anenhanced high temperature strength ductile iron alloy comprising carbonin an amount of 2.6 to 3.5% by weight, silicon in an amount of 3.7 to4.9% by weight, niobium in an amount of 0.45 to 1.0% by weight,manganese in an amount of 0.6% by weight or less, sulfur in an amount of0.02% by weight or less, phosphorus in an amount of 0.02% by weight orless, nickel in an amount of 0.5% by weight or less, chromium in anamount of 1.0% by weight or less, magnesium in an amount of 0.1% byweight or less, and the balance iron with 0.05% by weight or less of anyother single element, up to a combined total of 0.2% by weight of allsuch other elements. Typical for such other elements would be molybdenumand copper.

It is another object of the present invention to provide aheat-resistant ductile iron alloy that possesses high ductility and highcreep stress rupture properties. The alloy of the present invention withtargeted chemistry such as carbon at 3.0 to 3.3% by weight, silicon at3.75 to 4.25% by weight and niobium at 0.5 to 0.7% by weight should, atroom temperature, possess an ultimate tensile strength of 75,000 psi; a0.2% offset yield strength of 60,000 psi; and percent elongation of 10%.Additionally, the Brinell Hardness Number (BHN) of the cast materialmust fall within the range of 187 to 241 BHN, the BHN expressing thehardness of the alloy as the ratio of the pressure applied to a steelball forced in to the surface of the alloy to the surface area of theresulting indentation.

It is still another object of the present invention to provide theprocess for producing the enhanced high-temperature strengthhigh-silicon ductile iron alloy of the present invention.

The composition of the present invention has obtained these objects. Theproduct is formulated in accordance with the aforementioned percentagesby weight and, when formulated this way, there results an enhancedhigh-temperature strength ductile iron alloy.

Further objects and advantages of the alloy and process of the presentinvention will become apparent from the detailed description thatfollows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photographic image at 100× magnification showing themicrostructure of an etched sample of casting made with 0.56%molybdenum.

FIG. 2 is a photographic image at 500× magnification showing themicrostructure of the casting sample illustrated in FIG. 1.

FIG. 3 is a photographic image at 100× magnification showing themicrostructure of an etched sample of casting made according to thepresent invention with 0.46% niobium.

FIG. 4 is a photographic image at 500× magnification showing themicrostructure of the casting sample illustrated in FIG. 3.

FIG. 5 is a photographic image at 100× magnification showing themicrostructure of an etched sample of casting made according to thepresent invention with 0.67% niobium.

FIG. 6 is a photographic image at 500× magnification showing themicrostructure of the casting sample illustrated in FIG. 5.

FIG. 7 is a photographic image at 100× magnification showing themicrostructure of an etched sample of casting made according to thepresent invention with 0.94% niobium.

FIG. 8 is a photographic image at 500× magnification showing themicrostructure of the casting sample illustrated in FIG. 7.

FIG. 9 is a graph illustrating the ultimate tensile strength of thecasting sample illustrated in FIGS. 1 and 2 as compared to the ultimatetensile strength of the casting samples made according to the presentinvention with 0.46% and 0.67% niobium.

FIG. 10 is a graph illustrating the 0.2% method yield strength of thecasting sample illustrated in FIGS. 1 and 2 as compared to the 0.2%method yield strength of the casting samples made according to thepresent invention with 0.46% and 0.67% niobium.

FIG. 11 is a graph illustrating the elongation percentage of the castingsample illustrated in FIGS. 1 and 2 as compared to the elongationpercentage of the casting samples made according to the presentinvention with 0.46% and 0.67% niobium.

FIG. 12 is a graph illustrating the reduction of area of the castingsample illustrated in FIGS. 1 and 2 as compared to the reduction of areaof the casting samples made according to the present invention with0.46% and 0.67% niobium.

FIG. 13 is a photographic image at 100× magnification showing themicrostructure of an etched sample of casting made with 0.56% molybdenumfollowing heat soaking at 750° C. for 200 hours.

FIG. 14 is a photographic image at 500× magnification showing themicrostructure of the casting sample illustrated in FIG. 13.

FIG. 15 is a photographic image at 100× magnification showing themicrostructure of an etched sample of casting made according to thepresent invention with 0.46% niobium following heat soaking at 750° C.for 200 hours.

FIG. 16 is a photographic image at 500× magnification showing themicrostructure of the casting sample illustrated in FIG. 15.

FIG. 17 is a photographic image at 100× magnification showing themicrostructure of an etched sample of casting made according to thepresent invention with 0.67% niobium following heat soaking at 750° C.for 200 hours.

FIG. 18 is a photographic image at 500× magnification showing themicrostructure of the casting sample illustrated in FIG. 17.

FIG. 19 is a photographic image at 100× magnification showing themicrostructure of an etched sample of a cast turbocharger divider wallmade with 0.57% molybdenum.

FIG. 20 is a photographic image at 500× magnification showing themicrostructure of the casting sample illustrated in FIG. 19.

FIG. 21 is a photographic image at 100× magnification showing themicrostructure of an etched sample of a cast turbocharger divider wallmade according to the present invention with 0.60% niobium.

FIG. 22 is a photographic image at 500× magnification showing themicrostructure of the casting sample illustrated in FIG. 21.

FIG. 23 is a photographic image at 100× magnification showing themicrostructure of an etched sample of a cast turbocharger divider wallmade according to the present invention with ultra high silicon at4.67%, with 0.77% niobium and with higher end of chromium at 0.87%.

FIG. 24 is a photographic image at 500× magnification showing themicrostructure of the casting sample illustrated in FIG. 23.

FIG. 25 is a photographic image at 100× magnification showing themicrostructure of an etched sample of a cast turbocharger divider wallmade according to the present invention with ultra high silicon at4.45%, with 0.697% niobium and with lower end of chromium at 0.441%.

FIG. 26 is a photographic image at 500× magnification showing themicrostructure of the casting sample illustrated in FIG. 25.

DETAILED DESCRIPTION OF THE INVENTION

The alloy of the present invention is a high-silicon niobium ductileiron. As previously alluded to, niobium is an alloying element that iscurrently being used in the production of certain heat resistantstainless steels and aircraft engine parts. Niobium adjoins molybdenumin the periodic table and, as a result, these elements have very similaratomic weights. The industry standard that was used as a starting pointfor development of the niobium-add alloy of the present inventionspecifies carbon in an amount of 3.0 to 3.4% by weight, silicon in anamount of 3.75 to 4.25% by weight, molybdenum in an amount of 0.5 to0.7% by weight, manganese in an amount of 0.6% by weight or less, sulfurin an amount of 0.07% by weight or less, phosphorus in an amount of0.02% by weight or less, nickel in an amount of 0.5% by weight or less,magnesium in an amount of 0.08% by weight or less, and the balance iron.

The alloy of the present invention is an enhanced high temperaturestrength ductile iron alloy comprising carbon in an amount of 2.6 to3.5% by weight, silicon in an amount of 3.7 to 4.9% by weight, niobiumin an amount of 0.45 to 1.0% by weight, manganese in an amount of 0.6%by weight or less, sulfur in an amount of 0.02% by weight or less,phosphorus in an amount of 0.02% by weight or less, nickel in an amountof 0.5% by weight or less, chromium in an amount of 1.0% by weight orless, magnesium in an amount of 0.1% by weight or less, and the balanceiron with 0.05% by weight or less of any other single element, up to acombined total of 0.2% by weight of all such other elements. Typical forsuch other elements would be molybdenum and copper.

Strength and Ductility Testing

Certain tests are used in the art to provide critical design informationon the strength of materials, including materials such as the alloy ofthe present invention. For example, the high temperature progressivedeformation of a material at constant stress is called “creep.” In a“creep” test, a constant load is applied to a tensile specimenmaintained at a constant temperature, such as room temperature. Strainis then measured over a period of time. When data is plotted inaccordance with the measurements taken, a curve is formed whichtranslates into the strain rate or the creep rate of the material.Stress rupture testing is similar to creep testing except that thestresses used are higher than in a creep test and is always done untilthe material fails.

Such tests are necessary to determine performance characteristics ofalloys particularly when the alloys are intended, or specially designed,to be utilized in high temperature and high pressure systems. The gasturbine engine, for example, is one system that has several componentsthat tend to experience creep which, again, tends to occur under loadand at high temperatures. The alloy of the present invention has beenspecified by this inventor to be a heat-resistant ductile iron alloythat possesses higher ductility under conventional creep and stressrupture tests.

In the tests mentioned above, certain parameters are used to describestrength and ductility of a material, such as the alloy of the presentinvention. One strength parameter is the “ultimate tensile strength” (or“UTS”). The UTS is the stress limit at which the alloy actually breaks,with a sudden release of the stored elastic energy (i.e., by noise orheat) in the alloy. In accordance with the present invention, the alloyof the present invention should, at room temperature, possess a UTS of75,000 psi. This could also be represented by the pressure equivalent of75 KSI.

Another strength parameter is the “offset yield strength” of the alloy,which is determined by the amount of stress that corresponds to anintersection of the characteristic stress-strain curve mentioned aboveand a line drawn parallel to the elastic part of the curve, offset by aspecified strain. In the United States, the offset is usually specifiedas a strain of 0.2% or 0.1%. The alloy of the present invention should,at room temperature, possess a 0.2% offset yield strength of 60,000 psi,or 60 KSI.

Ductility is a qualitative, but subjective, property of an alloy. Themeasurement of a material's ductility can be used to indicate the extentto which the material can be deformed without fracture. One conventionalmeasure of ductility is the strain at fracture, which is usually calledthe “elongation.” This measurement is obtained after fracture by puttingthe specimen back together and taking the elongation measurement.Because an appreciable fraction of the deformation will be concentratedin a “necked” region of the tension specimen, the value of percentageelongation will depend on the length over which the measurement istaken. The alloy of the present invention should, at room temperature,possess a percent elongation of 10%.

Finally, the Brinell Hardness Number (BHN) of the alloy of the presentinvention must fit within the range of 187 to 241 BHN, the BHNexpressing the hardness of the alloy as the ratio of the pressureapplied to a steel ball forced in to the surface of the alloy to thesurface area of the resulting indentation.

Referring now to the figures, the alloy of the present invention willnow be illustrated by examples which are for the purpose of illustrationonly and are not in any way to be considered as limiting. Multiplecastings of each of the following melt samples were made. The firstsample was a high-silicon molybdenum ductile iron with 0.56% molybdenumby weight. The second sample was a high-silicon niobium ductile ironwith 0.46% niobium by weight. The third sample was a high-siliconhigh-niobium ductile iron with 0.67% niobium by weight. The fourthsample was a high-silicon ultra-high-niobium ductile iron with 0.94%niobium by weight. FIGS. 1 through 8 illustrate magnified images of eachof the samples that have been etched by nital, a dilute mixture ofnitric acid and alcohol.

More specifically, FIG. 1 illustrates, at 100× magnification, oneexample of a nital-etched microstructure of an alloy of known art. Thisfirst sample, identified as the high-silicon molybdenum ductile ironabove, was comprised, by weight, of 3.04% carbon, 3.94% silicon, 0.56%molybdenum, 0.39% manganese, 0.014% phosphorus, 0.006% sulfur, 0.039%magnesium, 0.072% nickel, and 0.015% niobium, the balance iron. At roomtemperature, the UTS of this high-silicon molybdenum alloy was 85.4 KSI,the 0.2% yield strength was 65.1 KSI, and the elongation percentage was18%. The hardness was 196-235 BHN. FIG. 2 illustrates, at 500×magnification, the microstructure shown in FIG. 1. The sampleillustrated in FIGS. 1 and 2 shows typical ferritic grain structure (10)and spheroidal graphites (12). Dispersed throughout this alloy sampleare structures (14) of pearlite. Pearlite is a mixture of ferrite andcementite which forms in the alloy as it cools. While the presence ofpearlite is desirable in cast ferrite alloys where pearlite is used as ameans of increasing the hardness of the alloy, it is also undesirable inapplications where higher ductility is desired since its presence alsoreduces ductility. With reduced ductility, the alloy, though harder, isalso more prone to fracture, particularly at high temperatures. As shownin FIG. 1, the use of molybdenum in the sample alloy in the amountspecified tends to produce pearlite amounts between 5% and 10%. Alsodispersed throughout the sample are ill-defined gray areas (16) ofintercellular complex carbides, which also adversely affect ductility.

FIG. 3 illustrates, at 100× magnification, one example of amicrostructure of an alloy according to the present invention which, byweight, was comprised of 3.08% carbon, 4.08% silicon, 0.03% molybdenum,0.37% manganese, 0.009% phosphorus, 0.005% sulfur, 0.035% magnesium,0.11% nickel, and 0.46% niobium, the balance iron. This example isreferred to as the “second sample” above and was identified above as ahigh-silicon niobium ductile iron. The UTS of this alloy was 89.4 KSI,the 0.2% yield strength was 70.6 KSI, and the elongation percentage was17%, all at room temperature. Its hardness was determined to be 196-235BHN. FIG. 4 illustrates, at 500× magnification, the microstructure shownin FIG. 3. The high-silicon niobium sample illustrated in FIGS. 3 and 4shows largely ferritic grain structure (20) and spheroidal graphites(22). Dispersed throughout the sample are black structures (24) ofpearlite. A shown, the use of niobium at 0.46% tends to reduce thepearlite amounts to less than 5%. Also dispersed throughout the sampleare ill-defined gray areas (26) of intercellular complex carbides andsmaller niobium carbide globules (28).

FIG. 5 illustrates, at 100× magnification, another example of amicrostructure of an alloy according to the present invention which, byweight, was comprised of 3.19% carbon, 3.92% silicon, 0.04% molybdenum,0.40% manganese, 0.009% phosphorus, 0.005% sulfur, 0.055% magnesium,0.0784% nickel, and 0.67% niobium, the balance iron. This example isreferred to as the “third sample” above and was identified above as ahigh-silicon high-niobium ductile iron. The UTS of this alloy was 83.5KSI, the 0.2% yield strength was 64.0 KSI, and the elongation percentagewas 19%, also all at room temperature. Its hardness was 196-235 BHN.FIG. 6 illustrates, at 500× magnification, the microstructure shown inFIG. 5. The high-silicon high-niobium sample illustrated in FIGS. 5 and6 shows largely ferritic grain structure (30) and spheroidal graphites(32). Dispersed throughout the sample are black structures (34) ofpearlite. A shown, the use of niobium at 0.67% tends to further reducethe pearlite amounts. Also dispersed throughout the sample areill-defined gray areas (36) of intercellular complex carbides andsmaller niobium carbide globules (38).

FIG. 7 illustrates, at 100× magnification, yet another example of amicrostructure of an alloy according to the present invention which, byweight, was comprised of 3.36% carbon, 3.91% silicon, 0.02% molybdenum,0.32% manganese, 0.013% phosphorus, 0.008% sulfur, 0.042% magnesium,0.04% nickel, and 0.94% niobium, the balance iron. This example isreferred to as the “fourth sample” above and was identified above as ahigh-silicon ultra-high-niobium ductile iron. At room temperature, theUTS of this alloy was 85.0 KSI, the 0.2% yield strength was 66.5 KSI,and the elongation percentage was 16%. Its hardness was 196-235. FIG. 8illustrates, at 500× magnification, the microstructure shown in FIG. 7.The high-silicon ultra-high-niobium sample illustrated in FIGS. 7 and 8shows largely ferritic grain structure (40) and spheroidal graphites(42). Dispersed throughout the sample are black structures (44) ofpearlite. A shown, the use of niobium at 0.94% tends to reduce thepearlite amounts even further. Also dispersed throughout the sample areniobium carbide globules (48). But note that there is no sign ofintercellular complex carbides in this sample.

As a general observation during testing of each of the above-mentionedspecimens, it was noted that the machining characteristics of thehigh-silicon niobium ductile iron of the present invention were superiorto those of the high-silicon molybdenum alloy. Also, the high-siliconniobium ductile iron of the present invention provided considerablyhigher ductility and creep stress rupture properties up to 800° C. thandid the high-silicon molybdenum ductile iron.

High Temperature Testing

The samples of the high-silicon molybdenum, the high-silicon niobium,and the high-silicon high-niobium alloys were each tested for theirrespective UTS, 0.02% offset yield, elongation percentage and “reductionof area” percentage values at temperature increments of 100° C. Thehigh-silicon ultra-high-niobium alloy was tested only at roomtemperature, as referred to above, and at 800° C., the extreme ends ofthis high temperature testing.

As shown in FIGS. 9 through 12, the performance characteristics of thefirst three samples are illustrated in graphical form based on testresults measured in 100° C. increments. Specifically, those include the0.56% molybdenum alloy, the 0.46% niobium alloy and the 0.67high-niobium alloy. FIG. 9 represents the UTS of those samples and FIG.10 represents the 0.2% yield strength of each. Recall that these valuesrepresent the relative “strength” of the alloys. FIG. 11 represents theelongation percentage and FIG. 12 represents the “reduction of areapercentage” values, also measured in 100° C. increments. These last twographs illustrate the relative “ductility” of the respective alloys. Itshould also be mentioned here that the “reduction of area percentage”value is a measure of the relative area of the “neck” of the specimen atthe point of fracture as compared to the area of the pre-stressedspecimen.

In each figure, the values of the 0.56% molybdenum alloy (110) are shownplotted against those of the 0.46% niobium alloy (120) and the 0.67%high-niobium alloy (130). As shown in FIGS. 9 and 10, it is evident thatthe “hardness” of the molybdenum alloy (110) is somewhat greater thanthat of either the niobium alloy (120) or the high-niobium alloy (130).However, it is also evident, in FIGS. 11 and 12, that the “ductility” ofthe molybdenum alloy (110) is substantially less than that of either theniobium allow (120) or the high-niobium alloy (130), particularly athigher temperatures.

High Temperature Soak Testing

Normalizing is a type of heat treatment applicable to ferrous metalsonly. Normalization involves the austenitizing of the ductile ironcasting, followed by cooling in air through a critical temperature. Thecasting is normalized by means of “soaking” the casting within a heatedenvironment for a pre-determined period of time. A ductile iron castingis normalized in order to break down carbides, to increase strength, andto remove the internal stresses that are induced within the casting andwhich are brought about by the casting process itself.

High temperature testing of the molybdenum alloy and the niobium alloysalso yielded specific average values of the strength and ductility testresults following heat soaking of the alloys at 750° C. for 200 hours.The samples were then allowed to cool to room temperature. The sampleswere then tested for strength and ductility at room temperature and at800° C.

At room temperature, the average UTS of the molybdenum alloy was 81.3KSI. The average UTS for the niobium alloy was 82.7 KSI and for thehigh-niobium alloy was 82.8 KSI. At room temperature, the average 0.2%offset yield of the molybdenum alloy was 62.5 KSI. The 0.2% offset yieldof the niobium alloy was 64.2 KSI and of the high-niobium alloy was64.5% KSI. Accordingly, the high temperature soaking resulted in theniobium addition alloys being slightly stronger at room temperature.

At room temperature, the average elongation percentage of the molybdenumalloy was 17%. The average elongation percentage for the niobium alloywas 18% and for the high-niobium alloy was also 18%. At roomtemperature, the reduction of area percentage of the molybdenum alloywas 24%. The reduction of area percentage of the niobium alloy was 26%and of the high-niobium alloy was 25%. Accordingly, the high temperaturesoaking also resulted in the niobium addition alloys being slightly moreductile at room temperature.

At 800° C., the average UTS of the molybdenum alloy was 5.8 KSI. Theaverage UTS for the niobium alloy was 5.2 KSI and for the high-niobiumalloy was 5.7 KSI. At 800° C., the average 0.2% offset yield of themolybdenum alloy was 4.0 KSI. The 0.2% offset yield of the niobium alloywas 3.5 KSI and of the high-niobium alloy was 3.8% KSI. Accordingly, thehigh temperature soaking resulted in the niobium addition alloysyielding slightly less strength at higher temperature than themolybdenum addition alloy.

At 800° C., the average elongation percentage of the molybdenum alloywas 57%. The average elongation percentage for the niobium alloy was 65%and for the high-niobium alloy was 61%. At 800° C., the reduction ofarea percentage of the molybdenum alloy was 60%. The reduction of areapercentage of both the niobium and the high-niobium alloys was 63%.Accordingly, the high temperature soaking also resulted in the niobiumaddition alloys being significantly more ductile at high temperatures.

FIGS. 13 through 18 illustrate magnified images of each of theheat-soaked samples that have also been nital-etched. More specifically,FIG. 13 illustrates, at 100× magnification, the first sample ofhigh-silicon molybdenum ductile iron. FIG. 14 illustrates, at 500×magnification, the microstructure shown in FIG. 13. Both microstructuresillustrated in FIGS. 13 and 14 at 100× and 500× show basically ferriticgrain structures (210) and spheroidal graphites (212) that are dispersedthroughout the sample. Note also the presence of intercellular complexcarbides (214), particularly in FIG. 14.

FIG. 15 illustrates, at 100× magnification, the heat-soaked high-siliconniobium ductile iron. FIG. 16 illustrates, at 500× magnification, themicrostructure shown in FIG. 15. The high-silicon niobium sampleillustrated in FIGS. 15 and 16 shows basically ferritic grain structures(220) and spheroidal graphites (222). Also dispersed throughout thesample are niobium carbide globules (228). Note the absence ofintercellular complex carbides in this sample.

FIG. 17 illustrates, at 100× magnification, the heat-soaked high-siliconhigh-niobium ductile iron. FIG. 18 illustrates, at 500× magnification,the microstructure shown in FIG. 17. The high-silicon high-niobiumsample illustrated in FIGS. 17 and 18 shows basically ferritic grainstructures (230) and spheroidal graphites (232). Also dispersedthroughout the sample are niobium carbide globules (238). Note theabsence of intercellular complex carbides in this sample as well.

Specific Product Testing

To further evaluate the abilities of the high-silicon niobium additionalloy of the present invention, two specially designed melts werecreated. A turbocharger was selected as the test casting due to itsaffinity for cracks propagating through the divider wall and tongue areawhen run on an engine test at high temperature. Sample batches ofhigh-silicon molybdenum alloy and high-silicon niobium alloy were used.The high-silicon molybdenum alloy had a chemical composition, by weight,of 3.12% carbon, 3.98% silicon, 0.57% molybdenum, 0.35% manganese,0.012% phosphorus, 0.007% sulfur, 0.041% magnesium, 0.09% nickel, 0.01%niobium, and the balance iron. The high-silicon niobium alloy had achemical composition, by weight, of 3.15% carbon, 4.17% silicon, 0.02%molybdenum, 0.32% manganese, 0.014% phosphorus, 0.009% sulfur, 0.039%magnesium, 0.14% nickel, 0.6% niobium, and the balance iron. Therelative hardness of the high-silicon molybdenum alloy ranged between217 BHN and 228 BHN. The high-silicon niobium alloy had a relativehardness of between 207 BHN and 228 BHN.

FIGS. 19 through 22 illustrate magnified images of each of theabove-referenced samples that have also been nital-etched. Morespecifically, FIG. 19 illustrates, at 100× magnification, the firstsample of the casting divider wall made with the high-silicon molybdenumductile iron, with 0.57% molybdenum. FIG. 20 illustrates, at 500×magnification, the microstructure shown in FIG. 19. The sampleillustrated in FIGS. 19 and 20 shows ferritic grain structure (310) andspheroidal graphites (312) along with well-defined black structures(314) of pearlite. Also dispersed throughout the sample are a largernumber of ill-defined gray areas (316) of intercellular complexcarbides.

FIG. 21 illustrates, at 100× magnification, the casting divider samplemade of high-silicon niobium ductile iron, with 0.60% niobium. FIG. 22illustrates, at 500× magnification, the microstructure shown in FIG. 21.The high-silicon niobium sample illustrated in FIGS. 21 and 22 showslargely ferritic grain structure (320) and spheroidal graphites (322)with very low percent, less than 2%, pearlite (324) with no sign ofintercellular complex carbides. Along with these structures are niobiumcarbide globules (328) dispersed throughout the sample, the presence ofwhich is good because such globules (328) will not break down duringuseful application of the structure.

Corrosion and Oxidation Resistance

As demonstrated above, testing of the niobium-add alloy proved that thealloy had a better microstructure containing very low, if any, pearliteand carbide content and that it had excellent ductility and creeprupture properties. It is known in the art that chromium added to aniron-based ductile alloy improves oxidation and corrosion resistanceproperties of the alloy. In view of that art, this inventor produced anultra high silicon niobium and chromium alloy to determine whether thoseproperties would be affected by the substitution of niobium formolybdenum in this type of alloy. The specification target that was usedas a starting point for development of the ultra high silicon niobiumand chromium alloy of the present invention specifies carbon in anamount of 2.8 to 2.9% by weight, silicon in an amount of 4.4 to 4.8% byweight, molybdenum in an amount of 0.05% by weight or less, niobium inan amount of 0.6 to 0.8% by weight, chromium in an amount of 0.75 to0.9% by weight, manganese in an amount of 0.4% by weight or less, sulfurin an amount of 0.02% by weight or less, phosphorus in an amount of0.04% by weight or less, nickel in an amount of 0.5% by weight or less,copper in an amount of 0.03 to 0.07% by weight, magnesium in an amountof 0.03 to 0.07% by weight or less, and the balance iron.

The heat of ultra high silicon niobium and higher end chromium alloythat was used in pouring turbocharger castings made according to thepresent invention is illustrated in FIGS. 23 and 24 which show magnifiedimages of the heat treated sample that has been nital-etched. The finalchemistry of this sample was, by weight, 2.79% carbon, 4.67% silicon,0.77% niobium, 0.87% chromium, 0.04% molybdenum, 0.34% manganese, 0.01%phosphorus, 0.01% sulfur, 0.03% magnesium, 0.08% nickel, and 0.05%copper, the balance iron. The mechanical properties of the fullyannealed heat treated sample yielded a UTS of 100 to 114 KSI, a 0.2%yield strength of 87 to 113 KSI, an elongation percentage of 9% and ahardness of 235 BHN. FIG. 23 illustrates the microstructure of this heattreated sample at 100× magnification. FIG. 24 illustrates, at 500×magnification, the microstructure shown in FIG. 23. The sampleillustrated in FIGS. 23 and 24 shows typical ferritic grain structure(410) and spheroidal graphites (412). Dispersed throughout this alloysample are chromium carbide structures (414) and niobium carbideglobules (418). Note the complete absence of pearlite and intercellularcomplex carbides in this sample.

Another heat of ultra high silicon niobium and lower end chromium alloywas used to pour turbocharger castings also made according to thepresent invention is illustrated in FIGS. 25 and 26 which show magnifiedimages of the second heat treated sample that has been nital-etched.FIG. 25 illustrates the microstructure of this heat treated sample at100× magnification. FIG. 26 illustrates, at 500× magnification, themicrostructure shown in FIG. 25. The sample illustrated in FIGS. 25 and26 again shows typical ferritic grain structure (420) and spheroidalgraphites (422). Dispersed throughout this alloy sample are chromiumcarbide structures (422) and niobium carbide globules (428). Note thecomplete absence of pearlite and intercellular complex carbides in thissample.

Conclusion Regarding Test Results

In the view of this inventor, the reason that the creep rupture test andductility of the alloy shows a much increased result when using niobiumover molybdenum is because of the fundamental microstructure differencesbetween the molybdenum and niobium additions. For example, in themolybdenum addition alloy, molybdenum tends to produce more pearliteamounts, those amount being between 5% and 10%. The niobium addition,however, tends to produce much less than 5% pearlite in themicrostructure. The molybdenum addition also tends to produce moreintercellular complex carbides than the niobium addition. The reason forthe occurrence of larger amounts of pearlite and intercellular complexcarbides in the molybdenum addition is that, after graphite noduleformation, the molybdenum tends to combine with the free carbon toproduce those items. In the niobium addition, niobium combines withcarbon and produces niobium carbides in a very fine globule shapethroughout the microstructure. The levels of pearlite and intercellularcomplex carbides in the molybdenum addition result is increased hardnessand reduced ductility of the alloy at room temperature and at hightemperatures along with lower creep stress rupture test results as isevident from the test results obtained. On the other hand, the endresult in the niobium addition alloy is a reduction in hardness and anincrease in ductility at room temperature and at high temperature alongwith higher creep stress rupture test results, also evident from thedata collected.

In the molybdenum addition alloy, when pearlite and intercellularcarbides break down at higher temperatures, there is an expansion in thecomponent which creates deformation and cracking in the casting.However, in the niobium addition alloy, there is little, if any, breakdown which results in less deformation and cracking in the casting. Thisis due to the very low presence of pearlite and intercellular carbidesin the niobium additions relative to the molybdenum addition and becausethe niobium carbides are very stable at high temperatures. Thestructural testing of these alloys also supports these test results.

The niobium-add alloy of the present invention also demonstratedenhanced performance properties when used in ultra high silicon chromiumand ultra high silicon and ultra high chromium applications forcorrosion and oxidation resistance.

Accordingly, it will be evident that there has been provided a new anduseful high-silicon niobium ductile iron alloy that demonstratesenhanced high temperature strength and performance characteristics and aprocess for producing this alloy.

1. An enhanced high temperature strength ductile iron alloy consistingof: carbon in an amount of 2.79 to 2.9% by weight, silicon in an amountof 4.17 to 4.67% by weight, niobium in an amount of 0.6 to 0.77% byweight, manganese in an amount of 0.6% by weight or less, sulfur in anamount of 0.02% by weight or less. phosphorus in an amount of 0.02% byweight or less, nickel in an amount of 0.08 to 0.14% by weight or less,chromium in an amount of 1.0% by weight or less, magnesium in an amountof 0.1% by weight or less, any other single element in an amount of 0.02to 0.05% by weight, up to a combined total of 0.2% by weight of all suchother elements, and the balance iron.
 2. A process for producing anenhanced high temperature strength ductile iron alloy consisting of thesteps of: providing carbon in an amount of 2.79 to 2.9% by weight,providing silicon in an amount of 4.17 to 4.67% by weight, providingniobium in an amount of 0.6 to 0.77% by weight, providing manganese inan amount of 0.6% by weight or less, providing sulfur in an amount of0.02% by weight or less, providing phosphorus in an amount of 0.02% byweight or less, providing nickel in an amount of 0.08 to 0.14% byweight, providing chromium in an amount of 1.0% by weight or less,providing magnesium in an amount of 0.1% by weight or less, providing0.02 to 0.05% by weight of any other single element, up to a combinedtotal of 0.2% by weight of all such other elements, providing thebalance iron, combining the elements, melting the combined elements, andair cooling the alloy in the form of an end-product.